The use of articles formed at least in part from a polymeric material is widespread. Foamed polymer materials in particular are used in a wide variety of applications because foamed polymer products possess desirable insulating, shock absorbing or strength to weight ratio properties. For example, the food service industry often uses foamed polystyrene containers thermoformed from a foamed polymer roll stock because these containers are strong, lightweight and possess good thermal insulating properties.
Unfortunately, the presence of static electricity on polymeric articles can limit the usefulness of the containers. Undesired static electricity can hinder separation of individual articles from a stack of nested articles and can cause the articles to collect dust or other fine particles present in the environment where the articles are manufactured or used.
Foamed polymer cups typify the foregoing problems. These cups usually are supplied to a food service vendor as a stack of several nested cups wrapped in a plastic sleeve. Static cling may only be a nuisance when foam cups are manually separated from a nested cup stack. However, when cup stacks are used in automated beverage vending machine applications, static charges can prevent reliable separating and dispensing of individual cups and cause the cups to collect dust prior to dispensing. While these effects are more pronounced with foam cups, the effects may occur when solid plastic cups are used, and to a lesser extent when paper cups are coated with one or more layers of solid polymeric materials.
Reliable cup separation is a critical prerequisite in vending applications. Automated vending machines typically employ vertical stacks of nested cups stored in a multi-position turret. When a drink is vended, a dispensing ring located at the bottom of a cup stack disengages the lowest cup from the stack. The disengaged cup traverses a chute or similar path by gravity to a cup fill area of the vending machine. Because gravity is the primary force causing the disengaged cup to travel to the cup fill area, and because a cup is a relatively light object, disengagement and travel of the cup can be substantially affected by electrostatic forces. These electrostatic forces can prevent the timely arrival of the cup in the cup fill area. If the cup is late to the cup fill area, incomplete cup fills can result. Even worse, electrostatic forces may prevent the cup from reaching the cup fill area, resulting in a customer watching his or her drink dispensing directly into the cup fill area drain. In addition to customer dissatisfaction, late or non-arrival of cups at the filling station often results in a demand for an unnecessary and expensive service call.
As a result of above discussed problems, vending service companies typically require that cup vending failures not exceed a failure rate of 1 in 1000 vending attempts, with some companies requiring that failure rates not exceed 1 in 100,000 vending attempts. These very demanding specifications have resulted in cup producers employing various mechanical and chemical methods in order to produce a reliably vended foam cup.
Mechanical efforts to improve the vendibility of foam cup attempt to minimize the contact area between adjacent nested cups, thereby reducing the chance that cups will fail to drop in a timely manner. For example, the Sweetheart Cup Company produces a TROPHY brand cup which includes a plurality of indentations in the lower cup sidewall. The indentations prevent the cups from nesting completely within each other, which minimizes contact of adjacent cup surfaces. While this mechanical method reduces friction between adjacent cups, some sidewall to sidewall contact between cups remains. Because static cling problems can occur when any part of a cup contacts any part of an adjacent cup, minimizing the area of sidewall contact does not eliminate electrostatic attraction between adjacent cups. Furthermore, cups with sidewall indentations increases the vertical stack height for a given number of cups which reduces the cup capacity of a given vending machine.
Electrostatic problems may also be reduced by applying chemical antistatic materials to foamed products. In food container applications, these antistatic agents typically take the form of organic compounds extruded or otherwise contained in either a typically hydrophobic polymeric foamed material layer or contained in solid material adhered to a polymeric foam material layer. In a typical foam cup application, an antistatic agent is combined with a resin used to fabricate a cup's foamed or solid polymeric material and subsequently migrates or "blooms" to the surface of the cup slowly during the perceived useful life of the cup. After blooming to the cup's surface, the antistatic agent interacts with atmospheric moisture to form a conductive surface which can dissipate static charges. In many instances, the use of such extruded antistatic compounds alone is insufficient to produce a reliably vended cup. The use of anti static agents as just described may be inadequate for any of several reasons. For example, because the antistatic agent "blooms" to the surface slowly or at an unpredictable rate, a discontinuous hydrophilic film may be formed that cannot uniformly dissipate a static charge from the object's surface. Alternatively, the foregoing method may fail either because the successful use of these compounds requires the presence of relatively high levels of ambient moisture which frequently are not present in air conditioned environments or under dry winter weather conditions, or because extruded antistatic materials remain predominantly within the polymeric material and therefore are not highly available at the surface of the polymeric material to minimize static accumulation during the cup production process. In addition, the choice of such anti static agents is limited in food container applications because of governmental regulations.
Although the antistatic agents discussed above may be applied directly to a surface of a cup, this process results in an uncontrolled loss of antistatic agent from the cup and interferes with the ability of the cup to durably retain a printed image during manufacturing or use. This later consideration is of substantial commercial importance because the sponsors of graphic information printed on a vending cup do not want their information to be displayed in a low quality manner or have the scuffed or smudged ink from the vending cup soil a user clothes or other objects.
Other types of non-blooming chemical antistatic compounds and methods to produce such products, such as those disclosed in U.S. Pat. No. 4,715,968, claim to function independently of ambient moisture conditions, and might be useful in products destined for automated vending applications, but these non-blooming compounds are not believed to meet governmental requirements for foodstuff-related applications.
Therefore, a need exists for polymeric products, including foamed polymeric products, that are relatively free from the effects of static electricity. The products should preferably retain their antistatic properties for extended periods of time, have no objectionable odors or taste as perceived by a consumer, and retain a well-defined printed image if a printed image is desirable.